Approaches are being developed that use over the air power transmission between a transmitter and the device to be charged. These generally fall into two categories. One is based on the coupling of plane wave radiation (also called far-field radiation) between a transmit antenna and receive antenna on the device to be charged which collects the radiated power and rectifies it for charging the battery. Antennas are generally resonant in order to improve the coupling efficiency. This approach suffers from the fact that the power coupling falls off quickly with distance between the antennas. So charging over reasonable distances (e.g., >1-2 m) becomes difficult. Additionally, since the system radiates plane waves, unintentional radiation can interfere with other systems if not properly controlled through filtering.
Other approaches are based on inductive coupling between a transmit antenna embedded, for example, in a “charging” mat or surface and a receive antenna plus rectifying circuit embedded in the host device to be charged. This approach has the disadvantage that the spacing between transmit and receive antennas must be very close (e.g. mms). Though this approach does have the capability to simultaneously charge multiple devices in the same area, this area is typically small, hence the user must locate the devices to a specific area.
As will be appreciated by a person having ordinary skill in the art, a near field communication (NFC) device may receive excessive power from a wireless power transmitter, which may result in undesirable heating of the NFC device. In addition, a rogue receiver might attempt to pick up power from the wireless power transmitter, thus affecting power delivery to a valid wireless power receiver and system efficiency.
Further, additional receivers or metal objects positioned within an associated charging region may detune a transmitter by reducing the self-inductance thereof. Variation in impedance looking into the transmitting coil of the transmitter may affect the performance of an associated driving amplifier. If the transmitting coil is significantly larger than the receiving coil, the coupling efficiency between the coils may suffer, which may impact charge time and cause potential thermal issues. Therefore, a higher efficiency coupling structure is desirable. It is also desirable to know the locations of one or more receivers within an associated charging region so that power may be diverted to the respective receivers.
A need exists for methods, systems, and devices for controlling field distribution of a wireless power transmitter. More specifically, a need exists for methods, systems, and devices for utilizing one or more parasitic antennas to control the field distribution of a wireless power transmitter.